In preparation for the armloss map I checked the calibration of the Y-Arm ITM and ETM OpLevs with the method originally described in https://nodus.ligo.caltech.edu:8081/40m/1247. I was getting a little confused about the math though, so I attached a document at the end of this post in which I work it out for myself and posteriority. Stepping through an introduced offset in the control filter for the corresponding degree of freedom, I recorded the change in transmitted power and the reading of the OpLev channel with the current calibration. One thing I noticed is that the calibration for ITM PIT is inverted with respect to the others. This can of course be compensated at any point in any readout/feedback chain, but it might be nice to establish some sort of convention where positive feedback to the mirror will increase the OpLev reading.
The calibration factors I get are within ~10% of the currently stored values. The table (still incomplete, need to relate to the current values) summarizes the results:
| Mirror |
DoF |
Current |
Relative |
New |
Y-Arm OpLev Calibration
| ETM |
PIT |
|
0.974 ± 0.029 |
|
| |
YAW |
|
1.077 ± 0.021 |
|
| ITM |
PIT |
|
-0.972 ± 0.020 |
|
| |
YAW |
|
0.920 ± 0.048 |
|
The individual graphs:
ETM PIT
 
ETM YAW
 
ITM PIT
 
ITM YAW
 
The math:
|